The long-term objective of this research program is to determine the function(s) of neurofilaments (NFs). NFs, the-major cytoskeletal structures large axons, are comprised of a triplet of proteins each of which contains a core region and a peripheral or COOH terminal domain of varying length. The latter is highly extended in the two larger NF subunits (NF-H and NF-M). Since the peripheral extensions of NF-H and NF-M contain multiple phosphorylation sites, including at least one multi-phosphorylation repeat, the phosphorylation of the peripheral domains of NF-H and NF-M is speculated to mediate important functions subserved by NFs putative functions ascribed to axonal NFs include: the regulation of fiber caliber, the interactions of NFs with one another and with other axonal organelles, the determination of the impulse conducting properties of axons that are thought to depend on the diameter of a given axon. For these reasons, this renewal application will: 1) define all potential phosphorylation sites on NF-M and NF-H; 2) identify kinases that phosphorylate NF-M and NF-H at physiological sites; 3) determine how these kinases modulate phosphorylation at these sites; 4) investigate the role of phosphorylation in the regulation of the diameter of axons. This research program will contribute new insights into those NF functions that are mediated by the differential phosphorylation of NFs. Further, the information obtained from these studies is likely to contribute towards a better understanding of the role that aberrant phosphorylation of neuronal cytoskeletal proteins may play in the pathogenesis of different neurodengenerative diseases such as Alzheimer's disease.